The present invention is directed to the development of improved anode materials for use in cathodic protection systems.
Anodes used in cathodic protection systems are hooked to a positive side of a power supply such as a DC rectifier, a battery charger or an AC to DC converter. The positive connection causes the anode material to undergo an anodic reaction wherein the anode material tends to dissolve and be consumed. An anode is a site where there is a loss of bonding electrons. The anode material ionizes (i.e., the material corrodes and an oxidation reaction occurs).
In the case of cathodic protection, it is desirable to select as the anode a material that is reluctant to corrode. At the same time, it is desirable that an alternative reaction occur such that anions (negatively charged ions such as hydroxide ions or chloride ions) are attracted to the surface of the anode material.
In cathodic protection, preferably negatively charged ions, which are attracted to the surface of the anode, release their electrons into the anode from which the power supply then pumps them into the cathode to keep the cathode from corroding. It is the imparting of electrical charge from the anode through the pump or DC convertor into the cathode that raises the energy level of the cathode structure (i.e., raises its energy level) and keeps it from corroding.
There are, then, two sides to the cathodic protection circuit, the cathode side and the anode side. Although there is a benefit to the cathode side, it is common to have destruction on the anode side, resulting from either the dissolving of the anode material or the corrosion of the electrolyte. If the anode material is gold or platinum, the primary reaction will not constitute dissolving gold or platinum. Rather, chloride or hydroxide ions from the electrolyte will be attracted to the surface. When they reach the surface, electrons are stripped off and the ions are converted into gas. In the case of chlorine ions the reaction is: EQU 2Cl.sup.- .fwdarw.Cl.sub.2 +2e.sup.-
In the alternative, the anode material itself may be dissolved. For example, in the case of an anode comprised nearly entirely of iron, the dominant reaction is the dissolving of the iron such that very little Cl.sub.2 gas is generated. The consumption rate of non-alloyed, non-catalyzed iron is about 20 pounds per ampere year.
Catalytic coatings are known for use on self healing valve metal substrates to improve the corrosion resistance of those substrates. For example, oxides of iridium, rubidium, ruthenium, and other precious metal oxide coatings can be applied to valve metal substrates such as the tantalum, niobium, and titanium family of metals ("valve metals"). The precious metal coatings used on such metal substrates are oxides. Hence, they are already corroded, but they are electrically conductive. Since they are already corroded, they become, to one degree or another, stable when the underlying material is intended to be operated as an anode. The substrates are driven as an anode and they tend to form an oxide layer which acts as an insulator. A protective oxide film is developed and does not permit any current discharge from the surface. It is a non-conductive oxide. But, if a conductive oxide is included on most of the surface of the substrate metal, and the oxide is scratched such that the metal substrate is exposed, then corrosion activity tends to occur there. The conductive oxide quickly conforms to an oxide of that metal at that location, and it stops discharging there. Discharge continues where the precious metal oxide coating is intact or where the metal oxide is conductive.
In addition to valve metals, graphite and high silicon iron are commonly used anode materials with relatively low consumption rates. They are much less expensive than valve metals and offer, in general, a consumption rate of about 1 to 2 lbs per ampere year in comparison to iron which is consumed at 20 lbs per ampere year. But in some instances it is desirable to have an even longer consumption rate to avoid periodic repairs to the cathodic protection system or to increase the current at which the system is run.
Thus, it has become desirable to improve the durability of the graphite and high silicon iron anode materials so that the consumption rate is improved. The improved anodes can be used for increased durations or be run at higher current densities.